This invention relates to a mechanical lock, and in particular, to a mechanical lock which incorporates features to prevent forced rotation of the cylinder when a mated key is not present in the key slot.
A mechanical ignition lock of a motor vehicle prevents operation of the ignition switch without the use of a properly mated key. In order to defeat a motor vehicle ignition lock, automobile thieves are known to grab the head of the cylinder of the lock and forcibly rotate the cylinder to its start position so as to actuate the ignition switch.
In order to deter the thief, hardened armor elements may be incorporated into the lock. Although they may be somewhat effective, hardened armor elements are cost prohibitive and may not prevent all attempts at forced rotation of the cylinder.
Other attempts to prevent forced rotation of the cylinder of a mechanical lock are shown in Milton U.S. Pat. No. 4,074,548 and Seidewand U.S. Pat. No. 4,074,547. Both the Milton and the Seidewand patents include a breakaway cylinder head which prevents forced rotation of the cylinder of the lock. As described, however, the cylinder heads in the Milton and Seidewand patents may also fragment in response to an operator's attempt to start an automobile with a properly mated key.
Therefore, it is a primary object and feature of the present invention to provide a mechanical lock wherein the cylinder head breaks away from the cylinder in response to forced rotation of the cylinder, but will not break away in conjunction with normal operation of the ignition switch with a properly mated key.
It is a further object and feature of the present invention to provide a mechanical lock wherein the breakaway cylinder head is simple and inexpensive to manufacture.
It is a still further object and feature of the present invention to provide a mechanical lock which prevents forced rotation of the cylinder by an automobile thief.
A mechanical lock is provided having a cylinder rotatably received within a sleeve. The cylinder defines an external cylindrical surface, longitudinal axis and a keyway disposed along the axis for slidably receiving a key therein. The cylinder includes a sidebar slot extending radially with respect to the axis, and a tumbler ward extending radially from the axis at a location angularly spaced from the sidebar slot. The sidebar slot has an open inner end which opens into the tumbler ward and an open outer end which opens to the external cylindrical surface.
The cylinder further includes a sidebar groove extending longitudinally parallel to the axis and having an open end which opens into the sidebar slot.
A sidebar slidably mounted within the sidebar slot for radial movement therein. The sidebar includes a tongue at its inner end and a radially outer end, and is movable between a first cylinder locking position where the outer end of the sidebar projects from the cylinder, and a second cylinder locking position wherein the outer end of the sidebar is retracted within the sidebar slot. Sidebar springs within the cylinder bias the sidebar inwardly toward the unlocking position.
A rib extends longitudinally from the sidebar parallel to the axis. The rib is receivable within the sidebar groove for radial movement therein. The rib further includes a radially outer end which projects from the cylinder when the sidebar is in the cylinder locking position.
A tumbler is disposed in the tumbler ward for sliding movement radially with respect to the longitudinal axis. The tumbler moves between a non-aligned position with the sidebar so as to hold the sidebar in its cylinder locking position, and an aligned position with the sidebar so as to permit the sidebar to move into its cylinder unlocking position. The tumbler further includes a notch of predetermined depth along one edge. Inner section of the key in the keyway moves the tumbler from the non-aligned position to its aligned position, and thereby aligns the notch with the tongue of the sidebar. This permits the sidebar to slide radially inwardly from the locking position to the unlocking position to prevent rotation of the cylinder in the sleeve. A tumbler spring within the cylinder biases the tumbler toward the non-aligned position.
A cylinder head extends circumferentially about a first end of the cylinder. The cylinder head includes an outer head portion and a frangible base portion connecting the outer head portion to the first end of the cylinder. The frangible ring portion fractures in response to a predetermined rotational force on the outer head portion.
A tab extends from the outer head portion of the cylinder head. The tab includes tab body portion and a frangible base portion connecting the tab body portion to the outer head portion. The frangible base portion fragments in response to a predetermined rotational force on the tab body portion.